Support for JTA 2.1 read-only transaction management

Following up on #35633, the `UserTransaction` read-only semantics are still in discussion. If they turn out to be stricter than Spring's read-only hint, we should only apply them when configured with an explicit `enforceReadOnly=true` flag at the Spring `JtaTransactionManager` level (similar to the same-named flag in `DataSourceTransactionManager`).